1. Field of the Invention
This invention relates to getting fresh water from sea water. Sea water is distilled at a pressure near the critical pressure.
2. Description of Related Art
The heat of vaporization of water is one of the major factors in the cost of distilling sea water.
Heating a liquid toits boiling temperature and supplying the latent heat of vaporization is required for distillation. A heat exchanger for recovery of the heat is required for economic operation. A heat exchanger needs a temperature difference to transfer heat. The outgoing fluid heats the incoming fluid. Heat is transferred until the incoming fluid reaches the temperature of the outgoing fluid. Then no further heat recovery is possible. For example, at atmospheric pressure it takes one cal/grm to heat water one degree Celsius. 80 cal/grm heats water from 20 degrees C. to 100 degrees C. 539 cal/grm converts the water to steam. 80 cal/grm from the outgoing water heats the incoming water to 100 degrees C. The system loses 539 cal/grm. Desalination of sea water at the critical point eliminates the heat of vaporization problem. The critical point is defined as the point beyond which there is no latent heat of vaporization and no other characteristic change which normally marks a change in phase. The critical pressure is the pressure at the critical point. At pressures at and above the critical pressure is the fluid can be heated from a lower temperature to a higher one without any discontinuity in the process. (Joseph H. Keenan, "Thermodynamics," John Wiley & Sons, Inc., 1941). Above the critical point water is always vapor. Keenan also states: "Properties change so rapidly near the critical point that it is difficult to maintain precision in experiment or analysis. In the present state of our knowledge there remain some uncertainties in the data given for saturation states within a few degrees of the critical point." The heat of vaporization approaches zero as the water temperature and pressure approaches the critical point (225.4 kg/cm2 and 374.1 degrees Celsius for pure water). FIG. 2 herein shows the latent heat of vaporization of pure (fresh) water at saturation pressure as a function of temperature. The heat of vaporization is 539 cal/grm at atmospheric temperature and drops at an ever increasing rate until it reaches zero at the critical temperature of 374.1 degrees C. The thermodynamic properties of water are from "Thermodynamic Properties of Steam" by Joseph H. Keenan and Frederick G. Keyes, John Wiley and Sons, Inc., New York, 1936.
The heat of vaporization is a problem only as it affects the cost of water. In 1991, water produced by the Santa Barbara desalination plant cost consumers about $1,900 per acre-foot. That compares with about $260 per acre foot for Colorado River water piped to Southern California and as little as $100 per acre foot for ground water. In 1991, water cost $534 per acre-foot per year in San Diego, Calif.
The Ewing Number (Ew) is a dimensionless measure of the performance of a distillation system. It is the heat of vaporization of the distilled fluid at atmospheric conditions times the mass rate of flow of the distilled fluid divided by the power input to the distillation system. The cost for one acre-foot for one year assuming electricity at $0.10/kw-hr is 77,000/Ew. On this basis, the following table shows the cost of distillation:
______________________________________ Cost Condition Ew $/acre-ft/yr ______________________________________ Atmospheric pressure without recovery .77 100,000 Atmospheric pressure with recovery 1.00 77,000 Operate at 28.1 kg/cm2 with recovery 1.24 62,000 Operate at 168.7 kg/cm2 with recovery 2.51 31,000 Operate at 225.0 kg/cm2 with recovery 16.0 4,800 Operate at 225.4 kg/cm2* with recovery 244.0 320 ______________________________________ *assumed critical pressure The average family uses onehalf acrefoot of water per year.
Commercial operation at the high pressure (225.4 kg/cm2) and high temperature (374.1 degrees Celsius) of the critical point is not a problem. In the early 1950's the first commercial supercritical-pressure steam turbine was developed. This commercial unit successfully operated at an inlet pressure of 316 kg/cm2 and an inlet temperature of 621 degrees Celsius. The flow for this unit was roughly equivalent to the water requirements of 4,000 families. The successful commercial operation of this unit shows that high flows, pressures and temperatures are not barriers to commercial conversion of sea water.
U.S. Pat. No. 1,904,716 to Thorssell (1916) recognizes that the heat of vaporization of water is one of the major factors in the cost of distilling sea water to get fresh water.
Thorssell states ". . . at the pressure and temperature corresponding to or near the critical point of the liquid . . . " There is no explanation of what this is or how it is determined.
Thorssell depends on the Joule-Thomson effect to condense the steam. He predicts a positive Joule-Thomson coefficient by extrapolation. FIG. 9 of Keenan and Keyes shows that the Joule-Thomson coefficient is zero at the critical point. The coefficient is positive for steam (vapor) and negative for water (liquid). The Joule-Thomson effect will not condense the steam at the critical point.
U.S. Pat. No. 2,520,186 to von Platen (1950) purports to take advantage of the zero heat of vaporization above the critical point by operating above the critical point. Von Platen claims that fresh water is obtained. Above the critical point there is no phase change to separate the salt free distillate from the salt laden concentrate. FIG. 5 herein shows the specific volume of saturated water and steam near the critical point.
U.S. Pat. No. 3,522,152 to Osdor (1970) notes that the high cost of sea water conversion remains and attributes this to the "squeeze" problem. The "squeeze" problem arises due to the variation of the specific heat of water, particularly near the critical point. FIG. 3 herein shows the enthalpy of pure water at 211 Kg/Cm2 and at 225.4 Kg/Cm2. The enthalpy increases at an increasing rate as the temperature approaches the critical temperature. This increasing rate is more clearly shown on FIG. 4 herein where the specific heat of water near the critical pressure is plotted versus temperature. A constant heat input will result in an ever decreasing rise in temperature below the critical point. Above the critical point, the reverse is true. A constant heat input will result in an ever increasing rise in temperature. The critical point is an unstable point. A thermal finite element computer model of counter flow heat exchangers for a constant unity specific heat and for a specific heat which varied as shown in FIG. 4 indicating the degradation in performance caused by the increase in specific heat. FIG. 6 herein shows the temperature difference between the incoming fluid and the outgoing fluid. The increase in specific heat near the critical point has a marked but finite effect. The ineffectiveness of the heat exchanger is the difference in total heat entering the heat exchanger and the total heat leaving the heat exchanger. A zero temperature difference between inlet and outlet flow is zero ineffectiveness and unobtainable. The model shows that unity specific heat results in a temperature difference of 5 degrees Celsius. The specific heat variation shown on FIG. 4 results in a temperature difference of 7 degrees Celsius. This is a change in loss of heat from 5 cal/grm to 7 cal/grm due to the peculiar specific heat property. The heat exchanger ineffectiveness is increased 40%, a serious but not fatal problem.
Osdor goes on to propose a complicated scheme to overcome the squeeze problem. Osdor proposes to operate ". . . in the vicinity of the critical pressure of substantially pure water." There is no explanation as to what the vicinity of the critical pressure is or how it is determined.
The following prior art does not take advantage of the reduction of the heat of evaporation at the critical point.
Russian patent 466,026 discloses "The correction unit provides information based on level sensor signals, which adjust steam flow rate to the heater." The inventor does not consider recovering the heat of vaporization.
U.S. Pat. No. 4,444,623 to Younger (1984) discloses a distillation process. The inventor does not consider recovering the heat of vaporization.
U.S. Pat. No. 3,433,717 to Loebel (1969) discloses a distillation process wherein some of the distillate and concentrate energy is transferred to the incoming sea water. The inventor does not consider recovering the heat of vaporization.
Canadian patent 480,505 to Rivera (1952) relates to method and apparatus for determining the purity of steam. A condensed sample of the steam is measured for conductivity. The inventor does not consider recovering the heat of vaporization.
U.S. Pat. No. 4,419,187 to Cheng, et al (1983) discloses a liquid-vapor interface in a thermal membrane distillation method. The inventor does not consider recovering the heat of vaporization.
U.S. Pat. No. 3,444,050 to Sundquist (1969) purports to distill sea water taking advantage of a "natural refrigerating phenomena." The pressure is "maintained within a limited range, just above atmospheric pressure." The inventor states "Curve separation in the temperature stratification column sets into motion a natural refrigeration effect. This effect is experienced when the fingers are quickly touched to the metal bottom of a pot of boiling liquid. While boiling continues, the bottom is felt to be cold. The significance of this phenomenon is that a heat flux is set up in a heated turbulent liquid. The heat flux is directed upward in opposition to the pull of gravity." For practical purposes heat flux is not affected by gravity. While the inventor may have felt cold, the metal bottom of the pot must have been hot, given the stated conditions.
Sundquist's FIG. 3 gives an equation which equates vaporization rate to a product of area, temperature to the 7/3 power, and the reciprocal of heat per unit mass. This is dimensionally inconsistent and cannot be a valid relationship.
Sundquist's FIG. 4 gives an equation for mass flow rate of vapor which if evaluated with any reasonable selection of values results in a prediction of negative mass flow rate of vapor. A positive input of sea water is predicted to result in a loss of distillate. This prediction casts doubt on the validity of the equation.
U.S. Pat. No. 3,736,235 to Sundquist (1973) purports to reduce the overhead clearance requirements by only pressurizing the heated hot distillate to prevent vapor formation. No change of the vapor pressure was proposed. The inventor does not consider recovering the heat of vaporization.
None of these patents has resulted in a practical, economical method of desalination. Only the first three propose to reduce the energy requirements by taking advantage of the reduced heat of vaporization near the critical point. They do not disclose a method for determining the actual pressure or condition of operation. This leaves a question as to how to determine the optimum operating temperature and pressure. There is also a question as to how to attain and maintain this optimum.